It is a common procedure to derotate the individual vertebra of the spine during a surgical scoliosis correction procedure or the like. This typically occurs once the spinal rods have been inserted into the pedicle screws that have been implanted. Such a surgical scoliosis correction procedure typically consists of 4 main steps:    1. Decortication of the spinal column. This consists of the removal of bone to loosen up the vertebral bodies so that they can be easily manipulated and straightened.    2. Insertion of the pedicle screws into the vertebra, as is done conventionally.    3. Insertion of the spinal rods and set screws into the pedicle screws to straighten the spine.    4. Rotation of the individual vertebral bodies along the spinal axis using the pedicle screws.
During a typical surgical scoliosis correction procedure it is not uncommon for the individual vertebra to be twisted along the spinal column. Although the spinal rods correct and straighten the spinal column, the individual vertebra can still be rotated along the axial plane. Typically, in conventional procedures, derotation tubes are placed over the heads of the implanted pedicle screws and are used to derotate each individual vertebra, which are then locked into position by tightening the set screw in each head body, locking the spinal rods into place. It should be noted that, as used herein, ‘rotation’ and ‘derotation’ are essentially synonymous, with the individual vertebra initially being rotated and subsequently being derotated via rotation of the associated pedicle screws.
This method works well if a surgeon is using monoaxial pedicle screws in the vertebrae. A monoaxial pedicle screw has a head body that is fixed to the threaded shaft with limited degrees of freedom, such that the whole vertebra can be derotated using the pedicle screw as a rigid lever arm. A problem arises, however, when the surgeon has placed a polyaxial pedicle screw with multiple degrees of freedom in the individual vertebra and attempts to derotate the individual vertebra; only the head body rotates as there is designed pivotal movement between the head body and the threaded shaft. There is no rigid lever arm by which to derotate the individual vertebra. Thus, in most cases, it is very difficult to derotate a vertebral body using a polyaxial pedicle screw, as the heads body moves or pivots, but the individual vertebra does not.
Thus, what is still needed in the art is an instrument that addresses this problem with polyaxial pedicle screws in such procedures.